Jan 19 (Reuters) - U.S. Attorney General Eric Holder and Lanny Breuer, head of the Justice Department's criminal division, were partners for years at a Washington law firm that represented a Who's Who of big banks and other companies at the center of alleged foreclosure fraud, a Reuters inquiry shows.

The firm, Covington & Burling, is one of Washington's biggest white shoe law firms. Law professors and other federal ethics experts said that federal conflict of interest rules required Holder and Breuer to recuse themselves from any Justice Department decisions relating to law firm clients they personally had done work for.

Both the Justice Department and Covington declined to say if either official had personally worked on matters for the big mortgage industry clients. Justice Department spokeswoman Tracy Schmaler said Holder and Breuer had complied fully with conflict of interest regulations, but she declined to say if they had recused themselves from any matters related to the former clients.

Reuters reported in December that under Holder and Breuer, the Justice Department hasn't brought any criminal cases against big banks or other companies involved in mortgage servicing, even though copious evidence has surfaced of apparent criminal violations in foreclosure cases.

The evidence, including records from federal and state courts and local clerks' offices around the country, shows widespread forgery, perjury, obstruction of justice, and illegal foreclosures on the homes of thousands of active-duty military personnel.

In recent weeks the Justice Department has come under renewed pressure from members of Congress, state and local officials and homeowners' lawyers to open a wide-ranging criminal investigation of mortgage servicers, the biggest of which have been Covington clients. So far Justice officials haven't responded publicly to any of the requests.

While Holder and Breuer were partners at Covington, the firm's clients included the four largest U.S. banks - Bank of America, Citigroup, JP Morgan Chase and Wells Fargo & Co - as well as at least one other bank that is among the 10 largest mortgage servicers.

A particular concern by those pressing for an investigation is Covington's involvement with Virginia-based MERS Corp, which runs a vast computerized registry of mortgages. Little known before the mortgage crisis hit, MERS, which stands for Mortgage Electronic Registration Systems, has been at the center of complaints about false or erroneous mortgage documents.

Court records show that Covington, in the late 1990s, provided legal opinion letters needed to create MERS on behalf of Fannie Mae, Freddie Mac, Bank of America, JP Morgan Chase and several other large banks. It was meant to speed up registration and transfers of mortgages. By 2010, MERS claimed to own about half of all mortgages in the U.S. -- roughly 60 million loans.

But evidence in numerous state and federal court cases around the country has shown that MERS authorized thousands of bank employees to sign their names as MERS officials. The banks allegedly drew up fake mortgage assignments, making it appear falsely that they had standing to file foreclosures, and then had their own employees sign the documents as MERS "vice presidents" or "assistant secretaries."

Covington in 2004 also wrote a crucial opinion letter commissioned by MERS, providing legal justification for its electronic registry. MERS spokeswoman Karmela Lejarde declined to comment on Covington legal work done for MERS.

It isn't known to what extent if any Covington has continued to represent the banks and other mortgage firms since Holder and Breuer left. Covington declined to respond to questions from Reuters. A Covington spokeswoman said the firm had no comment.

Several lawyers for homeowners have said that even if Holder and Breuer haven't violated any ethics rules, their ties to Covington create an impression of bias toward the firms' clients, especially in the absence of any prosecutions by the Justice Department.

O. Max Gardner III, a lawyer who trains other attorneys to represent homeowners in bankruptcy court foreclosure actions, said he attributes the Justice Department's reluctance to prosecute the banks or their executives to the Obama White House's view that it might harm the economy.

But he said that the background of Holder and Breuer at Covington -- and their failure to act on foreclosure fraud or publicly recuse themselves -- "doesn't pass the smell test."

Federal ethics regulations generally require new government officials to recuse themselves for one year from involvement in matters involving clients they personally had represented at their former law firms.

President Obama imposed additional restrictions on appointees that essentially extended the ban to two years. For Holder, that ban would have expired in February 2011, and in April for Breuer. Rules also require officials to avoid creating the appearance of a conflict.

Schmaler, the Justice Department spokeswoman, said in an e-mail that "The Attorney General and Assistant Attorney General Breuer have conformed with all financial, legal and ethical obligations under law as well as additional ethical standards set by the Obama Administration."

She said they "routinely consult" the department's ethics officials for guidance. Without offering specifics, Schmaler said they "have recused themselves from matters as required by the law."

Senior government officials often move to big Washington law firms, and lawyers from those firms often move into government posts. But records show that in recent years the traffic between the Justice Department and Covington & Burling has been particularly heavy. In 2010, Holder's deputy chief of staff, John Garland, returned to Covington, as did Steven Fagell, who was Breuer's deputy chief of staff in the criminal division.

The firm has on its web site a page listing its attorneys who are former federal government officials. Covington lists 22 from the Justice Department, and 12 from U.S. Attorneys offices, the Justice Department's local federal prosecutors' offices around the country.

As Reuters reported in 2011, public records show large numbers of mortgage promissory notes with apparently forged endorsements that were submitted as evidence to courts.

There also is evidence of almost routine manufacturing of false mortgage assignments, documents that transfer ownership of mortgages between banks or to groups of investors. In foreclosure actions in courts mortgage assignments are required to show that a bank has the legal right to foreclose.

In an interview in late 2011, Raymond Brescia, a visiting professor at Yale Law School who has written about foreclosure practices said, "I think it's difficult to find a fraud of this size on the U.S. court system in U.S. history."

Holder has resisted calls for a criminal investigation since October 2010, when evidence of widespread "robo-signing" first surfaced. That involved mortgage servicer employees falsely signing and swearing to massive numbers of affidavits and other foreclosure documents that they had never read or checked for accuracy.

Recent calls for a wide-ranging criminal investigation of the mortgage servicing industry have come from members of Congress, including Senator Maria Cantwell, D-Wash., state officials, and county clerks. In recent months clerks from around the country have examined mortgage and foreclosure records filed with them and reported finding high percentages of apparently fraudulent documents.

On Wednesday, John O'Brien Jr., register of deeds in Salem, Mass., announced that he had sent 31,897 allegedly fraudulent foreclosure-related documents to Holder. O'Brien said he asked for a criminal investigation of servicers and their law firms that had filed the documents because they "show a pattern of fraud," forgery and false notarizations.